Reciprocating subunit hypotheses for the mechanism of subunit interaction in oligomeric proteins are being tested via creation of hybrid enzyme molecules possessing one chemically modified (inhibited) and one normal subunit. Hybrid molecules are resolved from parental, totally modified and unmodified, species by isoelectric focusing, because the modification strategy involves creation of a discrete charge difference between modified and unmodified subunits. Kinetic comparison of purified hybrid and unmodified species is sufficient to rule out or to strongly support hypothesized mechanisms that call for subunits on the same enzyme molecule to turn over substrate synchronously but out of phase with one another.